Field of the Invention
The invention relates to a specific cell construction for electrochemical cells of the flow type, having minimized pressure drop in comparison to a conventional cell construction, and additionally exhibiting significantly improved flow distribution within the cell. With the electrochemical cell, moreover, a better power is achieved than with cells having a conventional cell construction.
Prior Art
Particularly in connection with changing energy generation, energy storage media are becoming increasingly important. Of particular interest are those examples which offer the possibility of being able to store large amounts of energy and to release it and take it up with high power. Preference here is given to technologies which store and release the energy with maximum efficiency, losing as little energy as possible in so doing, and thereby allowing cost-effective interim storage.
A technology much discussed for this purpose is that of redox flow storage media. A general representation of this technology from the prior art is shown in FIG. 1. In a redox flow storage medium, the energy is stored in the electrolyte in the form of metals, salts, or other chemical compounds, these compounds being present in liquid, dispersed, or dissolved form. The electrolytes are stored in external tanks 1a, 1b. For charging or discharging, the electrolytes are pumped through an electrochemical cell 2. In the electrochemical cell 2, through application of a voltage via a network connection 5 to the respective electrodes 3a, 3b, by oxidation and reduction reactions, electrical energy is converted into chemical energy during charging and is converted back into electrical energy during discharging. In a generalized form, the reactions taking place at the electrodes are as follows:

The electrochemical cell 2 consists of two half-cells, the anode side and the cathode side, in which the respective electrodes 3a, 3b are contained. The two half-cells are separated from one another by a permeable separating layer 4 for charge compensation during charging and discharging. For increase in energy, for example, a plurality of such individual cells may be brought together into cell stacks, or the active area of the individual cells can be increased.
The power capacity of an individual cell with a given active area is determined from the combination of cell voltage and current density, in other words the maximum power per unit area. This applies for both directions of the reaction, which takes place reversibly.
In order to achieve a maximum power density per cell, electrodes with a very high surface area are required. The power of a cell is determined by factors including the number of electrochemical reactions per unit time, and the geometric cell area [mol/(s*m2)]. Electrodes having a large surface area per unit geometric surface area therefore have many active centers at which the electrochemical reactions can proceed. Employed for this purpose in accordance with the prior art are three-dimensional porous electrodes, such as metal foams or high-porosity nonwoven carbon webs, for example, although other materials are also possible. The term “electrode” is equated in this patent application with the term “three-dimensional porous electrode”.
FIG. 2 shows a standard construction of a redox flow cell of this kind from the prior art. These electrodes 6a, 6b are integrated together with a permeable separating layer 4 into a cell frame, and a flow of the anolyte 8a and catholyte 8b passes through them in the X- or Y-direction during charging and discharging in each case, meaning that the oxidation or reduction reactions take place on the surface of the electrodes 6a, 6b. These electrodes 6a, 6b are delimited externally by side elements 7. In addition to delimitation externally, the side elements in a cell stack have the function of passing on the current from one cell into the next.
In the case of flow passing through in the X- or Y-direction, the state of charge (SOC) of the electrolyte decreases in the same direction on discharging and increases in the same direction on charging, and so the electrode, the side element, and the permeable separating layer see a different concentration of the respective active species in the total surface area. If too great a change in the SOC is then achieved per unit residence time of the electrolyte in the cell, then the firstly individual components, such as the permeable separating layer, the electrodes, and the side element, for example, are loaded differently at different locations; as a result, there may easily be irreversible damage to the respective components.
Furthermore, on charging, the power of the cell is always determined by the position on the electrode at which there is the highest SOC, since otherwise secondary reactions may easily take place.
Conversely, during discharging, the power capacity of the cell is determined by the position on the electrode at which there is the lowest SOC.
For a cell with a cell design of this kind, it is necessary for these reasons for only a very low change in charge to take place per unit residence time of the electrolyte in the cell. This means that for a given current density, the electrolyte must be pumped through the cell at a relatively high rate. Consequences of this, however, are an increasing pressure drop and hence increasing pump power, leading in turn to a sharp reduction in the system efficiency.
For uniform flow through the electrodes, furthermore, a relatively high volume flow rate is also necessary.
As already mentioned above, the overall efficiency of a redox flow storage medium of this kind is reduced not only by the electrochemical losses within the individual cells but also, in particular, by the pumping energy needed to convey the electrolyte through the cells. Most of the pumping energy here is needed in order to overcome the pressure gradient within the cell. This pressure gradient is caused on the one hand by the flow-impingement channels within the cell, but also, in particular, by the flow through the electrodes.
In order to reduce the pumping energy while maintaining uniform flow distribution of the electrolyte within the cell, a variety of approaches have been proposed.
International patent application WO 2012 022532 A1 (Cellstrom) describes an optimization of the distribution channel for improving the flow-related pressure drops and for uniform flow through the electrode.
European patent application EP 0814527 A2 (Sumitomo) also describes improvement to the distribution channels into the cell, and addresses the optimum ratio between cell height and cell width. It is said that, in particular, an increase in the cell height (length of the cell in flow direction) would lead to a reduction in the efficiency of the overall system because of increasing pumping power. There is also description to the effect that making the cell wider for the purpose of increasing power may lead to an uneven flow of electrolyte.
US patent specification U.S. Pat. No. 5,648,184 (Toyo) proposes reducing the pressure drop by providing the electrodes employed with a groove that is aligned with the flow of the electrolyte. The intention thereby is to reduce the pressure drop without affecting electrode power.
US patent specification U.S. Pat. No. 6,475,661 B1 (Chemieco) includes a proposal that the pressure drop can be reduced by applying flow profiles to the bipolar plate.
Subject matter of German laid-open specification DE 3401638 A1 (Hoechst) are electrolysis cells with liquid electrolytes and porous electrodes, in which the electrolyte enters parallel to the electrode surface and is forced by at least one constriction point to flow through the electrode at least partly parallel to the flow of charge.
Aaron et al. describe how very good electrochemical results can be achieved by means of what they call a “flow-by” cell construction. For these experiments, the redox flow cell used was a modified methanol fuel cell. Through an appropriate design of the flow channels, this “flow-by” technology permits a reduction in the pressure drop, but the design of the flow channels also always has a considerable effect on the power of the cell. The authors themselves note that, while the serpentine flow channels do result in good electrochemical power, this may also be associated with a high pressure drop. Furthermore, a concept of this kind harbors the risk of diffusion into the nonwoven carbon web that is used having a limiting effect where high current densities are a target [J. Power Sour. 206 (2012) 450-453].
Tian et al. describe the application of different flow channels within the electrodes. It was shown that this can lead to a considerable reduction in the pressure drop. Apparent, however, is a very uneven distribution of electrolyte within the cell, which leads to a reduction in the power capacity of the individual cell and may easily result, owing to different flow regimes, in secondary reactions such as evolution of oxygen or of hydrogen, for example [Rare Metals 30 (Spec. Issue) (March 2012) 16-21].
According to the prior art, the flow to redox flow cells arrives from one side of the electrodes, and the electrolyte flows through the electrodes in X- or Y-direction (see FIG. 2) and departs the cell again on the opposite side. Consequently, given that the electrodes used cause a high flow resistance for the electrolyte, there are unavoidably pressure drops, which make it very difficult or even impossible, both technically and economically, to upscale the cell in the X- and Y-directions simultaneously. On the one hand, such a high pressure drop in large cells would require technically costly and inconvenient designs, and on the other hand it would also represent a safety risk. Furthermore, the pumping power needed in order to overcome the pressure drop would reduce the overall efficiency of the system to an unacceptable degree.